Solutions of 0.5 M sodium bicarbonate (NaHCO
3
) are widely used to extract phosphorus (P) from soils as an index of its availability to plants and of environmental risk particularly using the method ...of Olsen et al. (1954). Extracts are traditionally made in the presence of activated carbon to minimize the presence of colloids that are not efficiently removed by centrifugation or filtration. Some extraction methods omit activated carbon, and a variety of post-extraction protocols have emerged for the P colorimetry. These variations have the potential to affect relations of test values with both P-fertilizer requirement and environmental risk for a variety of reasons. For example, the colloids include humic acid that precipitates under the acidic conditions required for the molybdate reaction, and humic acid contains P-esters and orthophosphate (PO
4
-P), at least some of which react with molybdate. We report experiments which show that (1) PO
4
-P in 0.5 M NaHCO
3
soil extracts containing colloids can be measured selectively and efficiently using on-line dialysis preceding colorimetry in a continuous-flow analyzer, and (2) these PO
4
-P values closely match those obtained in a blind interlaboratory comparison using 98 soil samples (R
2
= 0.99), where the independent data set was obtained following the original Olsen protocol using activated carbon, filtration, and manual colorimetry. The modified protocol is an efficient implementation of the Olsen-P test for high-throughput laboratories. Furthermore, extracting without activated carbon also opens the possibility of measuring total extracted-P, and estimating P in colloids by difference.
The manure fertilizer increases the phosphorus (P) saturation of soils and the colloidal P release to water bodies. Manure of different particle-sizes may have different effects on colloidal P ...release by soil, and to date there is limited knowledge on colloidal P release from soils amended with different size manures. We produced sheep micro- (SMicro) and nano-manure (SNano), and poultry micro- (PMicro), nano-manure (PNano) from bulk samples by wet fractionation method. The fractionation reduced the P contents of micro- and nano-manures, and enriched them in ash and calcium, iron (Fe), magnesium, and aluminum (Al) phosphate minerals compared with the bulk manures. The degree of P saturation (DPS) in Anthorsol and Cambisol was decreased (SMicro, 17.6 and 17.2 %; SNano, 14.5 and 13.3 % and PMicro, 19.0 and 19.7 mg kg−1; PNano, 17.0 and 14.3 mg kg−1) and released less colloidal P (SMicro, 3.12 and 3.78 mg kg−1; SNano, 3.01 and 3.56 mg kg−1 and PMicro, 3.34 and 3.92 mg kg−1; PNano, 3.21 and 3.65 mg kg−1) than the soils receiving the bulk manures. The decrease in colloidal P was correlated with less DPS in both soils amended with micro and nano manures. That is, the only measurable effect of manure particle size on colloidal P release from the amended soils was due to chemical fractionation during separation of the size fractions. It was suggested that nano and micro manures were the effective approach to reduce colloidal P release from manure amended soils.
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•Nano and micro manures were rich in phosphate minerals.•Nano manures reduced degree of phosphorous saturation and colloidal phosphorus release.•Nano manures increased P adsorption on soils.
Though rice is the predominant source of energy and micronutrients for more than half of the world population, it does not provide enough zinc (Zn) to match human nutritional requirements. Moreover, ...climate change, particularly rising atmospheric carbon dioxide concentration, reduces the grain Zn concentration. Therefore, rice biofortification has been recognized as a key target to increase the grain Zn concentration to address global Zn malnutrition. Major bottlenecks for Zn biofortification in rice are identified as low Zn uptake, transport and loading into the grain; however, environmental and genetic contributions to grain Zn accumulation in rice have not been fully explored. In this review, we critically analyze the key genetic, physiological and environmental factors that determine Zn uptake, transport and utilization in rice. We also explore the genetic diversity of rice germplasm to develop new genetic tools for Zn biofortification. Lastly, we discuss the strategic use of Zn fertilizer for developing biofortified rice.
Cadmium is a cumulative, chronic toxicant in humans for which the main exposure pathway is via plant foods. Cadmium-tolerant plants may be used to create healthier food products, provided that the ...tolerance is associated with the exclusion of Cd from the edible portion of the plant. An earlier study identified the cabbage (Brassica oleracea L.) variety, Pluto, as relatively Cd tolerant. We exposed the roots of intact, 4-week-old seedlings of Pluto to Cd (control ∼1 mg L⁻¹ treatment 500 μg L⁻¹) for 4 weeks in flowing nutrient solutions and observed plant responses. Exposure began when leaf 3 started to emerge, plants were harvested after 4 weeks of Cd exposure and the high Cd treatment affected all measured parameters. The elongation rate of leaves 4–8, but not the duration of elongation was reduced; consequently, individual leaf area was also reduced (P < 0.001) and total leaf area and dry weight were approximately halved. A/C ᵢ curves immediately before harvest showed that Cd depressed the photosynthetic capacity of the last fully expanded leaf (leaf 5). Despite such large impairments of the source and sink capacities, specific leaf weight and the partitioning of photosynthate between roots, stems and leaves were unaffected (P > 0.1). Phytochelatins (PCs) and glutathione (GSH) were present in the roots even at the lowest Cd concentration in the nutrient medium, i.e. ∼1 μg Cd L⁻¹, which would not be considered contaminated if it were a soil solution. The Cd concentration in these roots was unexpectedly high (5 mg kg⁻¹ DW) and the molar ratio of –SH (in PCs plus GSH) to Cd was large (>100:1). In these control plants, the Cd concentration in the leaves was 1.1 mg kg⁻¹ DW, and PCs were undetectable. For the high Cd treatment, the concentration of Cd in roots exceeded 680 mg kg⁻¹ DW and the molar –SH to Cd ratio fell to ∼1.5:1. For these plants, Cd flooded into the leaves (107 mg kg⁻¹ DW) where it probably induced synthesis of PCs, and the molar –SH to Cd ratio was ∼3:1. Nonetheless, this was insufficient to sequester all the Cd, as evidenced by the toxic effects on photosynthesis and growth noted above. Lastly, Cd accumulation in the leaves was associated with lowered concentrations of some trace elements, such as Zn, a combination of traits that is highly undesirable in food plants.
Mobile colloids impact phosphorus (P) binding and transport in agroecosystems. However, their relationship to P-lability and their relative importance to P-bioavailability is unclear. In soils ...amended with organic fertilisers, we investigated the effects of nano (NC; 1–20 nm), fine (FC; 20–220 nm), and medium (MC; 220–450 nm) colloids suspended in soil solution on soil P-desorption and lability. The underlying hypothesis is that mobile colloids of different sizes, i.e., NC, FC, and MC, may contribute differently to P-lability in soils enriched with organic fertiliser. NC- and FC-bound Pcoll were positively correlated with P-lability parameters from diffusive gradient in thin films (DGTA-labile P concentration, r ≥ 0.88; and DGTA-effective P concentration, r ≥ 0.87). The corresponding relations with MC-bound Pcoll are weaker (r values of 0.50 and 0.51). NC- and FC-bound Pcoll were also strongly correlated with soil P-resupply (r ≥ 0.64) and desorption (r ≥ 0.79) parameters during DGTA deployment, and the mobility of these colloids was corroborated by electron microscopy of DGTA gels. MC-bound Pcoll was negatively correlated with the solid-to-solution distribution coefficient (r = −0.42), indicating this fraction is unlikely to be the source of P-release from the solid phase after P-depletion from the soil solution. We conclude that NC and FC mainly contribute to regulating soil desorbable-P supply to the soil solution in the DGTA depletion zone (in vitro proxy for plant rhizosphere), and consequently may act as critical conditioners of P-bioavailability, whereas MC tends to form complexes that lead to P-occlusion rather than lability.
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•Nano and fine colloids in soil solution modulate soil phosphorus desorption and lability.•Nano- and fine-sized particles can carry desorbable P to the DGTA-depleted zone.•Medium colloids tend to form complexes that lead to P-occlusion rather than lability.
The agricultural use of manure fertilizer increases the phosphorus (P) saturation of soils and the risk of colloidal P (Pcoll) release to aquatic ecosystems. Two experiments were conducted to ...identify whether Pteris vittata plantation can decrease Pcoll contents in two soils (Cambisol and Anthrosol) amended with various manure P rates (0, 10, 25, and 50 mg P kg−1 of soil). The total Pcoll contents in manured soil without P. vittata were 1.14–3.37 mg kg−1 (Cambisol), and 0.01–2.83 mg kg−1 (Anthrosol) across manure-P rates. The corresponding values with P. vittata were 0.97–2.33 mg kg−1 (Cambisol) and 0.005–1.6 mg kg−1 (Anthrosol). Experimentally determined colloidal minerals (Fe, Al, Ca), colloidal total organic carbon, Mehlich−3 nutrients (Fe, Al, and Ca), and the degree of P saturation were good predictors of Pcoll concentrations in both soils with and without P. vittata plantation. In unplanted soils, P adsorption decreased and the degree of P saturation increased which released more Pcoll. However, P. vittata plantation decreased the Pcoll release and P loss risk due to the increase of P adsorption and reduced DPS in both soils. The P fractions (NaOH, NH4F, and HCl–P) contributed to increase the P pool in planted soils which enhanced the bioavailability of Pcoll and increased the P. vittata biomass. It suggested that P. vittata plantation was an effective approach to reduce Pcoll release from manure amended soils.
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•Soils with maximum P adsorption capacity reduce colloidal phosphorus release.•Pteris vittata plantation increase bioavailability of colloidal phosphorus.•Maximum Degree of P saturation promoted colloidal phosphorus release.
Phosphorus (P) is limiting nutrient in many soils, and P availability may often depend on iron (Fe) speciation. Colloidal iron phosphate (FePO
4coll
) is potentially present in soils, and we tested ...the hypothesis that phytate exudation by
Pteris vittata
might dissolve FePO
4coll
by growing the plant in nutrient solution to which FePO
4coll
was added. The omission of P and Fe increased phytate exudation by
P. vittata
from 434 to 2136 mg kg
−1
as the FePO
4coll
concentration increased from 0 to 300 mM. The total P in
P. vittata
tissue increased from 2880 to 8280 mg kg
−1
, and the corresponding increases in the trichloroacetic acid (TCA) extractable P fractions were inorganic P (860–5100 mg kg
−1
), soluble organic P (250–870 mg kg
−1
), and insoluble organic P (160–2030 mg kg
−1
). That is, FePO
4
-solubilizing activity was positive correlated with TP, TCA P fractions in
P. vittata
, TP in growth media, and root exudates. This study shows that phytate exudation dissolved FePO
4coll
due to the chelation effect of phytic acid on Fe; however, the wider question of whether phytic acid excretion was prompted by deprivation of P, Fe, or both remains to be answered.
Graphical abstract
Calcium phosphate (CaP) minerals may comprise the main phosphorus (P) reserve in alkaline soils, with solubility dependent on pH and the concentration of Ca and/or P in solution. Combining several ...techniques in a novel way, we studied these phenomena by progressively depleting P from suspensions of two soils (low P) using an anion-exchange membrane (AEM) and from a third soil (high P) with AEM together with a cation-exchange membrane. Depletions commenced on untreated soil, then continued as pH was manipulated and maintained at three constant pH levels: the initial pH (pHi) and pH 6.5 and 5.5. Bulk P K-edge X-ray absorption near-edge structure (XANES) spectroscopy revealed that the main forms of inorganic P in each soil were apatite, a second more soluble CaP mineral, and smectite-sorbed P. With moderate depletion of P at pHi or pH 6.5, CaP minerals became more prominent in the spectra compared to sorbed species. The more soluble CaP minerals were depleted at pH 6.5, and all CaP minerals were exhausted at pH 5.5, showing that the CaP species present in these alkaline soils are soluble with decreases of pH in the range achievable by rhizosphere acidification.
Zinc (Zn) is an important micronutrient in the human body, and health complications associated with insufficient dietary intake of Zn can be overcome by increasing the bioavailable concentrations in ...edible parts of crops (biofortification). Wheat (Triticum aestivum L) is the most consumed cereal crop in the world; therefore, it is an excellent target for Zn biofortification programs. Knowledge of the physiological and molecular processes that regulate Zn concentration in the wheat grain is restricted, inhibiting the success of genetic Zn biofortification programs. This review helps break this nexus by advancing understanding of those processes, including speciation regulated uptake, root to shoot transport, remobilisation, grain loading and distribution of Zn in wheat grain. Furthermore, new insights to genetic Zn biofortification of wheat are discussed, and where data are limited, we draw upon information for other cereals and Fe distribution. We identify the loading and distribution of Zn in grain as major bottlenecks for biofortification, recognising anatomical barriers in the vascular region at the base of the grain, and physiological and molecular restrictions localised in the crease region as major limitations. Movement of Zn from the endosperm cavity into the modified aleurone, aleurone and then to the endosperm is mainly regulated by ZIP and YSL transporters. Zn complexation with phytic acid in the aleurone limits Zn mobility into the endosperm. These insights, together with synchrotron‐X‐ray‐fluorescence microscopy, support the hypothesis that a focus on the mechanisms of Zn loading into the grain will provide new opportunities for Zn biofortification of wheat.
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